In pumps of the type described and in conventional rotary vane pumps, difficulties are encountered in pumping water vapour and vapours of volatile liquids and in pumping solvents (eg acetone).
With conventional pumps the difficulty with vapours arises because the substance being pumped tends to condense as it is pumped, so that output from the pump substantially decreases or ceases altogether. In theory, this problem could be overcome by running the pump at high temperatures, so that the vapour does not condense, but in conventional rotary vane pumps it has proved difficult to maintain correct clearances between relatively moving parts over extremes of temperature ranging from room temperature to 100.degree. C. or more.
In the case of solvents, a further problem is that they tend to dissolve in the oil normally used as the pumping liquid. It is known to use air to strip dissolved solvent out of the oil but the apparatus is cumbersome and the pumping rate is slow.
In a pump of the type described the fluid being pumped will normally be less dense than the rotating liquid ring, and having passed from the aperture into the ring will move rapidly to the centre of the rotating liquid. Thus the fluid accumulates in the central region of the pump from where it flows through a discharge passage which communicates with the central region of the pump.
Because this type of pump can be made without the need for very close tolerances in clearances in the main pumping chamber, the pump can be run at relatively high temperatures typically in the range 120.degree. C. to 160.degree. C., so that volatile fluids including water vapour do not tend to condense in the high pressure central region. Even so, it has been found in practice that when such fluids are being pumped there tends to be a significant reduction or cessation in output from the pump. It is believed that the problem may result from condensation of the vapour on cooler parts in the discharge line, with condensed droplets subsequently falling back into the main pumping chamber since the discharge line tends to lead vertically upwards. Although such droplets will boil off again, they can interfere with the pump operation in the meantime.
This type of pump also suffers from the problem that solvents tend to dissolve in the rotating liquid ring. Ideally a liquid is chosen in which the solvent will not dissolve, but it is not always possible to select such an alternative liquid since the choice of liquid is dictated by other considerations also particularly it must have a low vapour pressure at the temperatures of operation in order not to interfere with the operation of the pump and this tends to dictate the use of oils in which most solvents are to a greater or lesser extent, soluble. As the partial pressure of the dissolved solvent in the rotating liquid ring builds up, it begins to inhibit the movement of further solvent out through the aperture into the liquid, and in practice it is impossible to apply to a solvent a suction pressure below the partial pressure of the solvent which has built up in the rotating liquid ring.